June 2007 LSAT, III, #11

Section 3, Question #11, of the June 2007 LSAT presents yet another ridiculous argument. "Feathers from birds stuffed in the 1880s contain only half as much mercury as feathers recently taken from living birds, and mercury comes from fish, therefore today's fish must have more mercury than they did in the old days." No, your results do not indicate what you think they indicate. STFU... I will be back to deal with you in a minute.

Dear students:  I yell about this constantly, but no more than necessary. It's the most important thing I can teach you about LSAT Logical Reasoning. If you don't argue with the speaker when you're doing LR, you are doing it wrong. The argument above is asinine, and your primary task is to tell the speaker why he is full of shit. If you can tell the speaker why he is full of shit, the LSAT will become easier than you ever thought possible. If you can't, the test will remain a mystery.

So think about it for a second, before looking at the answer choices. Start by accepting the speaker's facts as true. It's a fact that the old birds contain less mercury in their feathers than modern birds do. And it's a fact that mercury in feathers comes from fish that are eaten by the birds. The part we're going to disagree with is the speaker's conclusion:  "These results indicate that mercury levels in saltwater fish are higher now than they were 100 years ago."

Why might the old birds have less mercury in their feathers? The speaker has proposed one explanation. It's possible that today's fish have more mercury. Sure. But that's not proven, nor is it the only explanation. If we can come up with another explanation, we can show the speaker to be full of shit. So,

  • How does the speaker know that mercury in feathers doesn't degrade over time? Maybe yesterday's fish had exactly the same amount of mercury in their feathers, or even more mercury in their feathers, but the mercury has evaporated in the last 130 years since the bird died.
  • Or maybe there's something in the "stuffing and preserving" process that removes mercury?
  • Or maybe seabirds from the 1880s didn't eat as much fish? Maybe they also ate crabs or bugs or something, that didn't have mercury?

And on, and on, and on. Any of the above, if true, would destroy the argument's assertion that today's fish must have more mercury in them than fish in the 1880s. We're 90% of the way to our answer.

So now, let's go ahead and look at the question. It says

"The argument depends on assuming that"

This is a Necessary Assumption question. For your reference, here's a post on what Assumption means, and here's a post on what Necessary Assumption means. In short, our job is to find an answer that must be true or else the argument will fail. I bet the correct answer will be related to one of our objections above. Here's what I mean.

Above, I predicted that if mercury in feathers evaporates over time, the argument makes no sense. So the argument has assumed, necessarily, that mercury in feathers does not evaporate over time.

Above, I predicted that if something in the stuffing and preserving process removes mercury from feathers, the argument makes no sense. So the argument has assumed, necessarily, that the stuffing and preserving process does not remove mercury from feathers.

Above, I predicted that if seabirds in the 1880s didn't eat fish, the argument makes no sense. So the argument has assumed, necessarily, that seabirds in the 1880s did eat fish.

Are you picking up what I'm laying down? For each one of my objections, I can phrase it in the form of a Necessary Assumption. Every weak spot in the argument can be phrased as an attack "1880s seabirds didn't eat fish!" or as a Necessary Assumption that defends against that attack:  "1880s seabirds did eat fish."

The point I'm trying to get at is this:  No matter what type of question you're dealing with, you can always benefit from attacking the logic of the argument.

This one is fully cooked... I'm extremely confident that we've already identified the correct answer. Let's jump into the answer choices:

A)  This answer would weaken the argument. It matches one of my attacks, above. On a Necessary Assumption question, we need to find an answer that helps the argument. Frequently, the correct answer will help the argument by defending against an attack. But it will never be the attack itself.

B)  Pollution? I don't see how pollution can be relevant to this argument. The point wasn't why there is more or less mercury in fish... the point was whether there is more mercury in today's fish. This isn't the answer.

C)  Nah. Feather growth is also irrelevant.

D)  This, like A, would weaken the argument. If this answer is true, it points to another potential explanation for why the 1880s feathers had less mercury. We need an answer that supports the author's given explanation.

E)  Yep, exactly. This matches our second prediction above, about the stuffing and preserving process. If this answer is untrue, the argument is in serious trouble. If the stuffing process does remove mercury from feathers, then how can the speaker possibly conclude that today's fish have more mercury? E is a Necessary Assumption that defends the argument from a devastating attack. So it's our answer.